Locking gas springs are well known and widely used in applications in which a member is moved by a spring force to a selected position and locked in that position. Perhaps the most well-known and common use of locking gas springs is in the seat support columns of office chairs. When most or all of the user's weight is off the seat and a control lever is moved to a release position, the spring force extends the spring and raises the seat. When the user sits on the seat and the gas spring is released, the user's weight overcomes the gas spring force, thus compressing the spring and lowering the seat. The user can adjust the seat to any desired height and by releasing the control lever when the desired height has been attained lock the seat in the desired position. Locking gas springs can be used for other adjustable installations, such as hospital beds, various forms of work tables and stands, and adjustable vehicle seat backs, such as for motor vehicles and aircraft.
Safety standards for the seat backs of motor vehicles and aircraft are readily met by locking gas springs used in tandem, that is, a locking gas spring on each side of the seat back. The necessity for tandem installations arises from the need to carry the very high forces imposed on the seat back in a crash, such as the rearward forces that occur when a motor vehicle is rear-ended and the seat occupant is forced back against the seat back by inertial forces or during the rebound phase of a front end collision when the occupant is forced back against the seat back by a restraint system. The tandem use of locking gas springs in vehicle applications requires synchronous control of the two springs by means of relatively costly control linkages.